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A few years ago I built myself a large router for cutting stand up paddle board blanks (pic bellow) however I have now got the need to start cutting other things so would like to start the process of designing and building a second machine.

The main thing the machine will be used for will probably be for cutting moulds for various things from aluminium with a high quality surface finnish. The moulds I will be cutting will be fairly large so feed rates are important. (also pictured being cut on a china machine I have been using however in surprisingly surface finnish isn't great)

At the moment I'm thinking 1220 x 600 cutting area with at least 100mm of useable z travel

I am thinking or running the gantry down the length of the table rather than width if possible so I can use the machine with full sheets.

obviously ball screws all round

then I would like to use the cs-labs cysimo again the same as on my last machine.

I'm open to suggestions, I would quite like to use ali extrusion again for the build as I'm set up for it and have quite a few bits kicking around however I also have a good amount (from memory) 90x90x2.5 box steel that I could passably use if needed for the build if I need to go the steel route.

when I can figure what exactly Is going to be best I will have a go at starting to draw it up on cad.

If your wanting high quality surface finish in aluminium then that setup is wrong way to go about it really. Unless the gantry is very heavily built it will vibrate along the long length and affect surface finish. Over building just costs more money so best avoided.

Would be much stronger traveling down the length with nice short stubby gantry.

How over built are we talking? the ability to take a full sheet of ply is worth something. If it wasn't the space restrictions I have I would be building a 8x4 machine in which case I guess I would be building the stronger gantry anyway.

I guess the only other way would be to have the gantry going down the length on the raised rails but have it only supported at ether end so a sheet can be passed under it?

Dean is right, if you expect perfect finish on aluminum you will have to overbuild like mine build. Ok maybe not so much but simplest will be 2x 100x100x3 box section welded together. Or something in the same range of strength at least. But also have to be fast so you can do HSM with light fast passes. But honestly i dont see how over that span you will go under 100kg gantry at least. My gantry is 180 kg for example with all mounted.

How over built are we talking? the ability to take a full sheet of ply is worth something. If it wasn't the space restrictions I have I would be building a 8x4 machine in which case I guess I would be building the stronger gantry anyway.

Charlie You said cutting Aluminium to high surface finish, ply is different thing altogether. Your first design will quite happily cut ply but aluminium is different matter. The Stiffness required goes up quite lot and to have high quality surface finish it goes to another level. Anything you can do to lessen resonance will help with surface finish so long hollow flexi beams are best avoided.
This means using thick walled heavily braced and stiffened design which opens up complete new set of issues regards ballscrews,servo, controllers etc which get expensive quickly.

If your spaced limited look at building vertical. You'll get 8x4 in the same space as your First machine. Something along the longs lines of this which I'm building at minute thou for high end aluminium I'd change the design slightly and do away with gantry sides.

I had wondered about vertical but it wont help much as I don't have much wall space spare to push up against, certainly for 8x4. I guess I will ether have to go for less quality on the aluminium or for go the ability for full sheets.